It is well known that low-back pain is one of the most frequently occurring and expensive disabling ailments, especially for patients in the 30-60 year age bracket. Although low-back pain syndrome is a very common occurrence, its diagnosis to this day is very difficult.
The vertebral column (spine) is a biomechanical structure composed primarily of ligaments, muscles, vertebrae and intervertebral discs. The biomechanical functions of the spine include (1) support of the body (trunk and appendages), which involves the transfer of the weight and the bending moments of the head, trunk and arms to the pelvis and legs, (2) complex physiologic motion between these body parts, and (3) protection of the spinal cord and the nerve roots.
The major regions of the spine are the cervical, thoracic, lumbar and sacral. The vertebrae increase in size and mass from the cervical to the lumbar regions. The increase in size of the vertebrae is directly related to an increased capacity for supporting larger loads. The lumbar region is therefore the major load bearer of the spine. However, this increase in load bearing capacity is paralleled by a decrease in flexibility. Because the lumbar region bears heavier loads than other regions of the spine, the lumbar trunk (low back structure) is more susceptible to strain and hence low-back pain.
The spine is comprised of different levels known as motion segment units. The lumbar spine is comprised of five motion segment units. The motion segment unit is the smallest component of the spine that exhibits kinematic behavior similar to that of the whole spine. The motion segment unit is capable of flexion, extension, lateral bending and translation. The components of each motion segment unit include two adjacent vertebrae and their apophyseal joints, the intervertebral disc and the connecting ligamentous tissue.
Many causes of low-back pain are attributed to the instability of the motion segment unit. Segmental instability is defined as "the loss of ability of the spine under physiologic loads to maintain relationships between vertebrae in such a way that there is neither damage nor subsequent irritation to the spinal cord or nerve roots, and, in addition, there is no development of incapacitating deformity or pain due to structural changes". In other words, instability is an abnormal response to applied loads characterized by motion in the motion segment unit beyond normal constraints. Excess motion can be abnormal in quality (i.e., abnormal coupling patterns) or in quantity (abnormal increased motion) or both. Excess motion results in damage to the nerve roots, the spinal cord, and other spinal structures.
The underlying causes of the structural changes in the motion segment unit leading to instability are trauma, degeneration, aging, disease (tumor, infection, etc.), surgery, or a combination thereof. It is known that a mechanically unstable motion segment unit can originate due to degeneration of the nucleus pulposus. A degenerate nucleus causes disc space narrowing, loss of viscoelastic properties and the subsequent transfer of compressive loads to the annulus fibrosus. The altered anatomic dimensions and subsequent abnormal response to loading can cause loss of pre-tension in the ligamenum flavum, and longitudinal ligaments, degeneration of the facet capsules (and possible subluxation) with a consequence of secondary degenerative osteoarthritis of the joints.
Spinal disorders requiring neural decompressive surgery can leave motion segment units unstable due to the removal of supporting structures of the joint. A severely unstable motion segment unit is most likely to be fused to insure post-surgical stability. The need to fuse the vertebrae of a motion segment unit is dependent on the pre-operative symptoms and clinical (radiographic) findings and on the outcome of the surgical procedure.
One effort at mechanically determining spinal instability is disclosed in "A Technique for Mechanical Assessment of the Interverebral Joint", Mark Lubin et al., Biomech. Sym. ADM vol. 43 (1981). A Cloward lamina spreader is fitted with a strain gauge and loading and unloading of force is conducted manually. The device disclosed in the aforementioned publication is disadvantageous because there is no recognition of the need to control the rate of displacement nor a means for doing so which enables precise measurements of relative stiffness of the motion segment unit. The motion segment unit is a viscoelastic structure and therefore its resistance to deformation is dependent on the loading rate. To date, there are no objective criteria for determining the degree of instability of the motion segment unit and whether spinal fusion surgery is necessary to relieve low-back pain in the patient.
It is therefore an object of the invention to provide an apparatus and method for measuring instability of a motion segment unit of the spine.
It is another object of the present invention to provide an apparatus and method for measuring instability of a motion segment unit of the spine wherein the loading and unloading of force can be conducted at a constant rate in order to enable precise measurements, which cannot be accomplished by prior art systems.
It is another object of the invention to provide an apparatus and method which serves as a diagnostic aid for determining the desirability of performing spinal fusion surgery at the time of the decompressive surgical procedure.
It is a further object of the invention to provide a method of measuring spinal instability.